Hereinafter, a “Q” or “q” prefix in a word of phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at the most fundamental levels. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (q-processor) uses the odd nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1s and 0s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually.
Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor (IBM is a registered trademark of International Business Machines corporation in the United States and in other countries.)
A superconducting qubit may include a Josephson junction. A Josephson junction is formed by separating two thin-film superconducting metal layers by a non-superconducting material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature—pairs of electrons can tunnel from one superconducting layer through the non-superconducting layer to the other superconducting layer. In a superconducting qubit, the Josephson junction—which has a small inductance—is electrically coupled in parallel with one or more capacitive devices forming a nonlinear resonator.
The information processed by qubits is emitted in the form of microwave energy in a range of microwave frequencies. The microwave emissions are captured, processed, and analyzed to decipher the quantum information encoded therein. For quantum computing of qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other types of superconducting quantum logic circuits, must not alter the energy states of the particles or the microwave emissions in any significant manner. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor structures that are used in such a circuit.
The illustrative embodiments recognize that a capacitor that is used in a superconducting quantum logic circuit, and particularly in a qubit—e.g. in conjunction with a Josephson junction—has to be fabricated according to this operational constraint. The presently used capacitor structure in a qubit is significantly larger in size than the size of the Josephson junction therein. FIG. 1 depicts a scaled view of a presently fabricated qubit. As can be seen, almost the entire area of qubit 100 is occupied by capacitor structure 102. Josephson junction 104 occupies a relatively insignificant area of qubit 100 as compared to the area occupied by capacitor structures 102.
The large size of the capacitor limits the number of qubits and other quantum readout circuitry that can be fabricated per die in a fabrication process. The illustrative embodiments recognize that a need exists for a method of fabricating a q-capacitor that is significantly smaller in the area occupied on the chip as compared to the presently used capacitor in quantum circuits, e.g., qubit 100. A q-capacitor is a capacitive device structure fabricated using superconducting material(s), where the capacitive structure is usable in a superconducting quantum logic circuit which stores and employs a single quantum of microwave energy during the operation cycle of the quantum logic circuit. Any absorption or dissipation of this energy, any spontaneous additions of energy, or fluctuations in the capacitance, arising in the q-capacitor, will degrade the circuit performance. An acceptable maximum threshold of these effects may be defined for a q-capacitor to function in the quantum logic circuit. A q-capacitor can be fabricated by using one or more superconducting materials on a silicon substrate in a semiconductor fabrication process, as described herein.